The present invention relates to a magnetic bearing device, and particularly to a controlled radial magnetic bearing device.
Magnetic bearings of a controlled type have recently been used for spindles of machine tools, turbo-molecular pumps, etc. which are driven at a high speed e.g. 30,000-40,000 r.p.m. or higher. As one of such magnetic bearings, a controlled radial magnetic bearing is known.
The bearing of this type generally includes an electromagnet for the X-axis and an electromagnet for the Y-axis provided in X- and Y-axis directions perpendicular to each other in a plane normal to the axis of the rotary shaft to magnetically support the rotary shaft; position sensors for X-axis and Y-axis are disposed on the X-axis and Y-axis, respectively, to detect any displacement of the rotary shaft in the X-axis and Y-axis directions from a reference position, and control circuits are provided for the X-axis and Y-axis for feeding the outputs of the position sensors back to the electromagnets and thereby controlling the outputs of the electromagnets.
With such a magnetic bearing device, the electromagnetic forces of the electromagnets for the X-axis and Y-axis are controlled by detecting any displacement of the rotary shaft in the X-axis and Y-axis directions by means of the position sensors, and then amplifying any differences from reference values preset in the control circuits, and then phase-compensating them, and then amplifying the outputs.
In case such a conventional controlled magnetic bearing device is used with a spindle of a turbo-molecular pump, for example, a fin rotor mounted on the spindle increases the inertia moment, so that as the speed of rotation increases, the gyroscopic moment sometimes generates a reverse precession in a direction reverse to the rotation of the spindle (as shown in FIG. 2).
Such a precession can be restrained by increasing the gain of the control system and the dynamic rigidity. However, the increase of gain has a disadvantage of requiring a higher voltage for control and causing increased vibration during rotation.